1. You may prefer to save them for your souvenir scrapbook.
2. If your vacation is longer than three days in the theme parks, and you would need a new Disney ticket, it is more cost effective to buy that new ticket for the entire vacation for each person and not use the old tickets.
Example. You will be there for six days. If you bought a 4 day ticket expecting to use two of the old tickets, you save a grand total of about seven dollars compared with buying a 6 day ticket and not using the old tickets.
Nowadays, post-2004 tickets used more than 13 days ago may no longer be upgraded. Pre-2005 tickets that had any usage at any time may not be upgraded. Child to adult, or non-magnetic format to magnetic, exchanges can always be done.